Fiber lasers, fiber optics for communication systems, and other systems for light delivery, such as in medical, industrial and remote sensing applications, are often handling high powers, namely, optical powers up to several Watts in a single fiber, waveguide or other light delivery channel. When these high specific intensities or power per unit area are introduced into systems, many of the thin film coatings, optical adhesives, detectors and even bulk material, are exposed to light fluxes beyond their damage thresholds and are eventually damaged. Another issue of concern in such high power systems is laser safety, where well-defined upper safety limits are allowed. These two difficulties call for a passive device that will switch off the power propagating in, e.g., a fiber, when the power exceeds the allowed intensity. Such a switching device can be placed either at the input of a sensitive optical device, or at the output of a high power device such as a laser or an optical amplifier, or integrated within an optical system.
In the past, there have been attempts to realize an optical safety shutter, mainly for high power laser radiation and high pulsed power radiation; special efforts were devoted to optical sights and eye safety devices. The properties on which these prior art solutions were based included: (1) self-focusing or self-defocusing, due to a high electric field induced index change through the third order susceptibility term of the optical material, and (2) reducing the optical quality of a gas or a solid transparent insert, positioned at the cross-over spot of a telescope, by creating a plasma in the cross over point to absorb light. These techniques are described in U.S. Pat. Nos. 3,433,555 and No. 5,017,769. In the U.S. Pat. No. 3,433,555, the plasma is created in a gas where the gas density is low (lower than solids and liquids), and the density of the plasma created by the gas is low as well, limiting its absorption to the medium and far infrared part of the light spectrum. This device is not absorbing in the visible and near infrared regions and cannot protect in these regions of the spectrum. U.S. Pat. No. 5,017,769 describes using a solid insert in the cross over point; this transparent insert is covered with carbon particles on its surface, enhancing the creation of the plasma on the surface at lower light intensities, and here the plasma density is high, since it starts from solid material. The dense plasma absorbs visible as well as infrared light, which is an advantage, and the device is equipped with multiple inserts on a (motorized) rotating wheel to expose a new, clean and transparent part after every damaging pulse. The two devices described in U.S. Pat. Nos. 3,433,555 and 5,017,769 are large in their volume, work in free space and require high pulsed powers and thus are less applicable to continuous lasers, to high repetition rate sources or to optical communication devices where powers are lower and fiber or waveguide (in line) devices are preferred.
Passive devices were proposed in the past for image display systems. These devices generally contained a mirror that is temporarily or permanently damaged by a high power laser beam impinging on it, damaging the mirror by distortion or evaporation. Examples of such devices are described in U.S. Pat. Nos. 6,384,982, 6,356,392, 6,204,974 and 5,886,822. The powers needed in these devices are in the range of pulsed or very energetic CW laser weapons and not in the power ranges for communication or medical devices. The distortion of a mirror by the energy impinging on it is very slow and depends on the movement of the mirror's large mass as well as the energy needed to cause the move. The process of removal of reflecting coatings from large areas is also slow, since the mirror is not placed in the focus, where power is spatially concentrated. Another passive device was proposed in U.S. Pat. No. 6,218,658, where two adjacent materials were used (the first material was heat absorbing while the second material was heat degradable). When these materials are inserted into the light beam, the first is heated and transfers its heat to the second, which degrades its transparency or reflectivity due to the high temperature. This process is relatively slow, since heat transfer times are slow, and in many cases not sufficiently fast to intercept the beam before damage occurs to objects along the optical line. In addition, the process of temperature-induced degradation does not provide enough opacity to efficiently prevent damage in high-power spikes that are a known phenomenon in laser fiber amplifiers. Another approach, using a nanostructure, made of nanoparticles in flake shape, was used by Donval et al. in U.S. Pat. No. 7,162,114 and Japanese Patent No. 4376632, to create an optical switch for fibres or an optical fuse.
Better, more opaque, faster-reacting and easier-to-manufacture solutions are needed. The present invention provides a solution accordingly.